(1) Field of the Invention
This invention relates to a novel process for producing a zirconium-based alloy. In particular, the invention relates to a zirconium-based alloy having high corrosion resistance to high temperature vapors.
(2) Description of the Prior Art
Zirconium-based alloys have excellent corrosion resistance and an extremely small neutron absorbing cross-section and are therefore used for the structural members of atomic power plants such as the fuel cladding pipes, fuel channel boxes, fuel spacers, and so forth.
These structural members are always exposed to neutrons as well as the high temperature, high pressure water or vapor inside the reactor for an extended period of time, so that oxidation proceeds to such an extent that the plant operation is significantly affected. Hence, the corrosion resistance of these zirconium-based alloys must be improved. If the alloys have low corrosion resistance, the working ratio of the plants operations will drop.
At the same time, the life-time of the fuel rods has been extended (the combustibility of the rods has been increased) in recent years and severer requirements have been imposed on the corrosion resistance of fuel cladding pipes.
Typical examples of zirconium-based alloys used for the structural members of atomic power plants include "Zircaloy-2" (a zirconium alloy containing about 1.5% by weight Sn, about 0.1% Fe, 0.1% Cr and about 0.05% Ni) and "Zircaloy-4" (a zirconium alloy containing about 1.5% by weight Sn, about 0.2% Fe and about 0.1% Cr). The production processes for these Zr-based alloys are disclosed in U.S. Pat. No. 3,865,635 and U.S. patent application Nos. 632,478 (1975) and 552,794 (1975).
U.S. Pat. No. 3,865,635 discloses a process in which the alloy is heated to a temperature within the .beta. phase range and is then subjected to cold working and annealing before final cold working. In accordance with this process, however, after solid solution treatment is carried out in the .beta. phase, final cold working is effected, followed by annealing. Accordingly, the crystal grains of the resulting alloy are large, and the tensile strength as well as the toughness are low. Since the alloy after the solid solution treatment has a high hardness, the subsequent cold working step is difficult to practice and this also results in the difficulty in further reducing the crystal grain size.
U.S. patent application Nos. 632,478 (1975) and 552,794 (1975) disclose a heat-treating process in which after the starting blank is shaped into the form of the final product, it is heated to a temperature within the .beta. phase range or within the (.alpha.+.beta.) phase range and then quenched. In accordance with these processes, however, deformation is likely to occur because the blank is quenched from a high temperature and hence, mold working must be carried out after the heat-treatment. However, the heating and cooling steps of a blank in the form of the final product are difficult to control and the problem of residual stress develops besides the problems of the oxidation of the surface and deformation due to thermal stress. To solve these problems, the oxide film must be removed and the deformation corrected by .beta.-annealing
In any of the abovementioned heat-treating processes an elongated blank must be heat-treated, and an extended period of time is necessary for the heat-treatment if a zone heat-treating process is employed in which the heating and cooling of the blank are carried out locally and continuously.